Conventionally, commercial differential pressure transmitters have used a differential pressure transmitter that includes a pressure sensor chip that uses a sensor diaphragm that outputs a signal in response to a differential pressure. This differential pressure transmitter is configured so that the pressures that are applied to a high-pressure side and a low-pressure side of a pressure bearing diaphragm are transmitted to the respective sides of the sensor diaphragm through a sealed liquid, as a pressure transmitting medium, where strain on the sensor diaphragm is detected as, for example, a change in a resistance value of a strain resistance gauge, where this change in resistance value is converted into an electric signal that is read out.
Such differential pressure transmitters are used, for example, to measure the height of a fluid surface in, for example, a high temperature reaction tower in an oil refinery by detecting the differential pressure between two locations at different has in a closed tank that stores the fluid that is being measured.
FIG. 8 illustrates schematically a conventional differential pressure transmitter. The differential pressure transmitter 100 is structured with a pressure sensor chip 1 having a sensor diaphragm (not shown) incorporated in a meter body 2. The sensor diaphragm in the pressure sensor chip 1 is made from silicon, glass, or the like, and the strain resistance gauge is formed on the surface of the diaphragm, which is formed as a thin plate. The meter body 2 is made from a main unit portion 3, made out of metal, and a sensor portion 4, where barrier diaphragms (pressure-bearing diaphragms) 5a and 5b, which form a pair of pressure-bearing portions, are provided on the side faces of the main unit portion 3, and the pressure sensor chip 1 is incorporated in the sensor portion 4.
In the meter body 2, between the pressure sensor chip 1 that is incorporated into the sensor portion 4 and the barrier diaphragms 5a and 5b that are provided in the main unit portion 3, pressure transmitting mediums 9a and 9b, such as silicone oil, are sealed into connecting ducts 8a and 8b that connect the pressure sensor chip 1 and the barrier diaphragms 5a and 5b through connecting, respectively, through pressure buffering chambers 7a and 7b that are separated by a large-diameter center diaphragm 6.
Note that the reason why the pressure medium, such as silicone oil, is necessary is because it is necessary to separate the sensor diaphragm, which has the sensitivity to the stress (pressure), from the pressure-bearing diaphragm, which is resistant to corrosion, in order to prevent foreign material within the measurement medium from adhering to the sensor diaphragm and to prevent corrosion of the sensor diaphragm.
In this differential pressure transmitter 100, a first measurement pressure Pa from a process is applied to the barrier diaphragm 5a and a second measurement pressure Pb, from the process, is applied to the barrier diaphragms 5b, as illustrated schematically for the proper operating state in FIG. 9(a). As a result, the barrier diaphragms 5a and 5b dislocate and the pressures Pa and Pb that are applied thereto are conveyed through the pressure transmitting mediums 9a and 9b through the pressure buffering chambers 7a and 7b that are separated by the center diaphragm 6, to the respective sides of the sensor diaphragm of the pressure sensor chip 1. As a result, the sensor diaphragm of the pressure sensor chip 1 undergoes dislocation corresponding to the differential pressure ΔP between these two transmitted pressures Pa and Pb.
In contrast, when, for example, an excessively large pressure Pover is applied to the barrier diaphragm 5a, the barrier diaphragm 5a undergoes a large dislocation, as illustrated in FIG. 9(b), and thus the center diaphragm 6 undergoes deformation so as to absorb the excessively large pressure Pover. Moreover, when the barrier diaphragm 5a tightly contacts the bottom face (an excessive pressure protecting face) of a recessed portion 10a of the meter body 2 so that that dislocation is constrained, this prevents the transmission of any differential pressure ΔP in excess of that to the sensor diaphragm through the barrier diaphragm 5a. Similarly, when an excessively large pressure Pover is applied to the barrier diaphragm 5b, then, in the same manner as when an excessively large pressure Pover was applied to the barrier diaphragm 5a, then when the barrier diaphragm 5b tightly contacts the bottom face (the excessive pressure protecting face) of the recessed portion 10b of the meter body 2 so that that dislocation is constrained, this prevents the transmission of any differential pressure ΔP in excess of that to the sensor diaphragm through the barrier diaphragm 5b. The result is that this prevents breakage of the pressure sensor chip 1 by the application of the excessively large pressure Pover, that is, this prevents in advance breakage of the sensor diaphragm in the pressure sensor chip 1.
In this differential pressure transmitter 100, the pressure sensor chip 1 is enclosed within the meter body 2, thus making it possible to protect the pressure sensor chip 1 from the external corrosive environment, such as the process fluids. However, because the recessed portions 10a and 10b are provided in order to constrain the dislocation of the center diaphragm 6 and the barrier diaphragms 5a and 5b, in a structure to protect the pressure sensor chip 1 from the excessive pressure Pover thereby, the dimensions thereof are unavoidably larger.
Given this, a first stopper member and a second stopper member are provided in the pressure sensor chip, where recessed portions of the first stopper member and the second stopper member face the respective surfaces of the sensor diaphragm to thereby prevent excessive dislocation of the sensor diaphragm when an excessively large pressure is applied, in a structure that has been proposed for preventing breakage/destruction of the sensor diaphragm thereby. See, for example, Japanese Unexamined Patent Application Publication 2005-69736 (“the JP '736”).
FIG. 10 illustrates schematically a pressure sensor chip that uses the structure shown in the JP '736. In this figure, 11-1 is a sensor diaphragm, 11-2 and 11-3 are first and second stopper members that are bonded with the sensor diaphragm 11-1 interposed therebetween, and 11-4 and 11-5 are pedestals to which the stopper members 11-2 and 11-3 are bonded. The stopper members 11-2 and 11-3 and the pedestals 11-4 and 11-5 are formed from silicon, glass, or the like.
In the pressure sensor chip 11, recessed portions 11-2a and 11-3a are formed in the stopper members 11-2 and 11-3, where the recessed portion 11-2a of the stopper member 11-2 faces one face of the sensor diaphragm 11-1, and the recessed portion 11-3a of the stopper member 11-3 faces the other face of the sensor diaphragm 11-1. The recessed portions 11-2a and 11-3a have curved surfaces (spherical surfaces), following the dislocation of the sensor diaphragm 11-1, and, at the apexes thereof, pressure guiding holes 11-2b and 11-3b are formed. In the pedestals 11-4 and 11-5 as well, pressure guiding holes 11-4a and 11-5a are formed at positions corresponding to the pressure guiding holes 11-2b and 11-3b of the stopper members 11-2 and 11-3.
When this type of pressure sensor chip 11 is used, when an excessively large pressure is applied to one face of the sensor diaphragm 11-1, causing the sensor diaphragm 11-1 to undergo dislocation, the entirety of the dislocated face is supported and stopped by the curved surface of the recessed portion 11-3a of the stopper member 11-3. Moreover, if an excessively large pressure is applied to the other face of the sensor diaphragm 11-1, causing the sensor diaphragm 11-1 to undergo dislocation, the entirety of the dislocation face is supported and stopped by the curved surface of the recessed portion 11-2a of the stopper member 11-2.
As a result, when an excessively large pressure is applied to the sensor diaphragm 11-1 excessive dislocation is prevented, making it possible to increase the excessive pressure-protected operating pressure (durability) by effectively preventing accidental damage to the sensor diaphragm 11-1 through the application of an excessively large pressure, through making it so that there are no concentrated stresses at the peripheral edge portions of the sensor diaphragm 11-1. Moreover, in the structure illustrated in FIG. 8, it is possible to achieve miniaturization of the meter body 2 through eliminating the center diaphragm 6 and the pressure buffering chambers 7a and 7b and guiding the measurement pressures Pa and Pb directly from the barrier diaphragms 5a and 5b to the sensor diaphragm 11-1.
However, in the structure of the pressure sensor chip 11 illustrated in FIG. 10, the stopper members 11-2 and 11-3 are bonded over the entire surfaces of the peripheral edge portions 11-2c and 11-3c to the respective faces of the sensor diaphragm 11-1. That is, the peripheral edge portion 11-2c of the recessed portion 11-2a of the stopper member 11-2 is caused to face one of the faces of the sensor diaphragm 11-1 and the entire region of the peripheral edge portion 11-2c that is thus facing is bonded directly to the one face of the sensor diaphragm 11-1. In addition, the peripheral edge portion 11-3c that surrounds the recessed portion 11-3a of the stopper member 11-3 is caused to face the other face of the sensor diaphragm 11-1, and the entire region of the peripheral edge portion 11-3c that is thus facing is bonded directly to the other face of the sensor diaphragm 11-1.
In the case of such a structure, when a pressure is applied to one side, causing the sensor diaphragm 11-1 to flex, the neighborhood of the edge of the sensor diaphragm 11-1 (the position surrounded by the dotted line in FIG. 10) on the side on which the pressure is applied, where the greatest tensile stress is produced, is in a state that is constrained on both sides, and thus there is a concentrated stress at this place, and thus there is a problem in that it is not possible to secure the expected durability.
Furthermore, when there is a mismatch, in manufacturing, between the opening sizes of the recessed portions 11-2a and 11-3a of the stopper members 11-2 and 11-3, there will be a positional mismatch in the locations of the constraints on the sensor diaphragm 11-1, and the effect of this may be a remarkable increase in the concentration of stresses. In this case, this combines with any concentration of stresses accompanying any adhesion defect in the sensor diaphragm 11-1, with the risk that this can further reduce durability.
The present invention is to resolve problems such as described above, and an aspect of the present invention is to provide a pressure sensor chip able to secure the expected durability by reducing the stresses produced by the constraints on the sensor diaphragm.